Process for the thermal treatment of light metals and light metal alloys



Patented Jan. 5, 1937 UNITED STATES PATENT OFFICE PROCESS FOR THE THERMAL TREATMENT OF LIGHT METALS AND LIGHT METAL ALLOYS tion of Germany No Drawing. Application October 31, 1935, Serial No. 47,721. In Germany October 31, 1934 7 Claims. (01. 148-131) Our invention relates to the thermal treatment of light metals.

Melt baths consisting of alkali nitrates or alkali nitrites, which have been heated to temperatures of from 300 to 400 C. or 500 to 550 C., have hitherto been used for the thermal treatment, for example annealing and normalizing, of light metals, such as aluminium and its alloys. The use of baths as aforesaid is attended with the risk of decompositions and explosions. When using such baths explosion phenomena are particularly liable to occur when the iron container commences to react with oxygen evolved from the salt bath or if relatively large quantities of organic substances gain access to the bath or if the saltbath, owing to leaks in the container,

comes into contact with combustible substances,

such as oil, coke, soot and the like.

Baths, which predominantly consists of thio- I cyanogen compounds of the alkali metals, have also already been proposed for the heat treatment of magnesium and magnesium alloys. The use of these relatively costly melt baths is limited to the narrow temperature range of- 150 to 300 0., since the thiocya-nates decompose at higher temperatures.

Cheap melt baths consisting of alkali metal chlorides or mixtures of alkali metal chlorides and alkaline earth metal chlorides are not suitable for use in the heat treatment of light metals, such as aluminium and the like, since they exert a strongly corrosive action on the light metal article under treatment.

According to the present invention baths, which contain as their main constituents alkali metal halides, such as sodium chloride, sodium bromide, potassium chloride, lithium chloride, or mixtures thereof or mixtures of alkali metal halides and alkaline earth metal halides, such as calcium chloride, barium chloride, strontium chloride and the like, in conjunction with basic reacting compounds, for example oxides, hydroxides or carbonates of the alkali metals or alkaline earth metals, are used for the thermal treatment of light metals and light metal alloys.

According to this invention it has been found that owing'to the presence of alkaline compounds the light metals canbe extensively and, if necessary practically completely, protected against attack by the halides.

As basic-reacting constituents of the melt bath there may be employed, in addition to those mentioned above, basic-reacting phosphates, tungstates, borates and the lik", as well as oxides, such as magnesium oxide, aluminium oxide and the like. The protective efiect may be obtained by the presence of one basic-reacting compound or several compounds having a basic reaction.

Melt baths of the aforesaid type may be employed for the heat treatment of light metals within wide temperature ranges, for example between 300 and 550 C. It is possible by suitably selecting and controlling the components of the salt bath to adjust the same to the desired melting point, which must of course be below the annealing and normalizing-temperature of the light metal articles under treatment. The basic-reacting protective substances are preferably used in such quantities, that as extensive a protective action as possible is obtained. In general relatively small additions of basic-reacting substances, such as caustic alkalies, carbonates and the like, are sufficient. When employing alkali carbonates the maximum quantity should in general not exceed 30%. Relatively large quantities of carbonates, for example quantities of 50% and more, cannot be used, because the melting point of the bath would be so far increased that it could no longer be used for the heat treatment of light metals. v

The basic-reacting compounds may be added to the melt bath during its preparation or in the course of the heat treatment, for example in doses calculated to counteract the natural exhaustion of the bath. The basic-reacting protective substances may also be caused to form in the bath itself, for example by adding compounds, which by decomposition or interaction are capable of forming the desired basic-reacting protective sub stances. Examples of additions as aforesaid are alkali cyanides as well as waste material from light metals or light metal'alloys, for example.

magnesium waste. The formation of the desired basic-reacting protective substances may be promoted by suitable procedures, for example by temporarily raising the bath temperature, intro ducing oxygen, for example by blowing through air, or adding peroxides, such as calcium peroxide.

When treating aluminium sheet at temperatures' of 530 C. in a salt bath consisting of a mixture of alkali chloride and alkaline earth chloride and composed of 48 parts CaClz, 31 parts BaClz and 21 parts NaCl, 102 mgms. of aluminium per square decimetre were dissolved in minutes. bonate only 6 mgms. of aluminium were dissolved in the same unit of time (30 minutes).

Particularly favourable results may be obtained by using baths, which contain potassium chloride and sodium chloride in addition to one or After adding 10% of barium carmore alkaline earth chlorides and, for example, alkaline earth carbonate as basic-reacting compound.

A melt bath of the composition:

when treating aluminium alloys, for example aluminium alloys known under the trade name of Bondur which contains, besides aluminium, 4.5% copper, 0.4% silicium, 0.7% manganese and 0.5% magnesium, showed absolutely no attack at 510 C. after 30 minutes duration. When employing a similar bath but without barium carbonate, 52 mgms. 01' metal were dissolved at the same working temperature (510 C.).

Baths of the aforesaid type may be adusted to melting points between 450 and 550 C. For the preparation of baths having lower melting points, for example from about 300 C. upwards, lithium chloride is with advantage co-employed. A melt of potassium chloride and lithium chloride in the approximate proportion of 4:6, which at about 380C. dissolved 151 mgms. of aluminium foil per square decimetre in 30 minutes, after adding 6% oi. sodium carbonate, for example, only dissolved 31 mgms.

What we claim is:

1. A process for the thermal treatment of a metallic substance selected from the group consisting of light metals and light metal alloys, particularly aluminium and its alloys which comprises treating the said metallic substance in a molten salt bath having a melting point between 300 and 550 C. which contains as its main constituents atleast one alkali halide and at least one basic-reacting compound, the proportion oi the said basic reacting compound bein8 such that the metallic substance is at least ex-' tensively protected from attack by the halide,

. metallic substance selected from the group consisting 01' light metals and light metal alloys, particularly aluminium and its alloys which comprises treating. thesaid metallic substance in a molten salt bath having a melting point between 300 and 550 C. which contains as its main constituents atleast one alkali halide, at least one alkaline earth halide and at least one basicreacting compound, the proportion 01' the said basic-reacting compound. being such that the metallic substance is at least extensively protectedfrom attack by the halides, said basicreacting compound constituting less than 30% 01 the constituents.

3. A process as claimed in claim 1, wherein the basic-reacting compound is selected from the group consisting oi alkalies and alkaline earths.

4. A process as claimed in claim 2, wherein the basic-reacting compound is selected from the group consisting of alkalies and alkaline earths.

5. A process as claimed in claim 1, wherein the alkali halide is selected from the group consisting 01' sodium chloride, potassium chloride and lithium chloride.

6. A process as claimed in claim 2, wherein the alkali halide is selected from the group consisting of sodium chloride, potassium chloride and lithium chloride.

metal alloys. 1

KLAUS BONAI'H.

CARL 

